1. Field of the Invention
This invention relates to a pneumatic tire for high speed vehicles and more particularly to an improvement in a carcass cross-sectional shape of an extremely low profile tire for racing cars having an aspect ratio within a range between 0.25 and 0.75 and a tread portion whose outer surface is substantially cylindrical.
2. Description of the Prior Art
Such kind of tire is required, in the first place, to have highly stable maneuverability, excellent accelerating and braking characteristics and better durability.
Contrary to tires used in general for passenger cars or trucks, the above mentioned kind of tire is not used for a long time under a mild speed condition, but is used for a relatively short time under an extremely high speed condition. As a result, if the above mentioned characteristics of the tire do not arrive at a given level, the tire could not satisfactorily run. In addition, if the construction and material of the tire is out of order, the tire is subjected to intensive mechanical and thermal stimulus, thereby inducing a breakage failure of the tire within an extremely short time.
Recent progress and development of the techniques related to automobiles result in a significant improvement in high speed performance of the tire. As a result, measures of improving the above mentioned tire characteristics to a higher level are now urgently in demand.
Various means have heretofore been proposed to improve the running performance of such kind of tire. In this case, it is, generally important to take into the consideration how to increase the ground contact area of the tire during running of the tire under load.
In the first place, it has been proposed to ensure a large ground contact area of the tire by deforming the tire such that the tire becomes wide in width and the cross-sectional plane of tire becomes flat. Such means, however, is limited by the construction of the car body.
Secondly, it has been proposed to ensure a large ground contact area of the tire in the peripheral direction thereof by making the outer diameter of the tire extremely large. Similar to the first proposed means, this second proposed means is also limited by the construction of the car body.
Finally, it has been proposed to ensure a large ground contact area by reducing the rigidity of the tread region and hence enlarging the ground contact area with the aid of the carcass construction. The above finally proposed means can obtain the large ground contact area by making the inclined angle of the carcass ply cords with respect to the equatorial plane of the tire large. In this way, the finally proposed means can successfully enlarge the ground contact area, but the carcass ply cord angle is large with respect to the circumferential direction of the tire, so that the effect of restraining the tread in the circumferential direction of the tire by means of the ply cords becomes reduced. As a result, the centrifugal force produced when the tire runs at a high speed causes the center region of the tread portion to be bulged, thereby degrading the durability of the tire. In addition, the transverse rigidity of the side portion becomes reduced, and as a result, the tire tends to be easily unstable when the tire is subjected to the exterior force during its turning movement.
In addition, when the tire runs along a straight road, there is a risk of the tire being subjected to standing wave phenomenon by the presence of the bulge protruded from the tread due to the centrifugal force and also by the reduced transverse rigidity.